Lewis Hamilton "wasn't satisfied" despite all the success he has had with Mercedes, according to one of the team's former top bosses.
The Silver Arrows became the team to beat as they adapted best to new car regulations in 2014. And they made the most of it by launching a dynasty which saw them take eight consecutive constructors' titles.
Hamilton was the spearhead of that success. Nico Rosberg and Valtteri Bottas also made significant contributions, but six of the Brit's seven drivers' championship successes came in that period.
Also playing a key role, albeit behind the scenes, was James Vowles. He was promoted through the ranks to become their main strategy chief and a key ally to team principal Toto Wolff.
During his time with Mercedes, Vowles worked with all the above drivers and more, including the legendary Michael Schumacher. But Hamilton stands out to the now Williams chief above the rest because of his constant drive and ambition despite all his success.
He said: "As with all humans, no two are the same. They're very different in characteristics. The one that impressed me year-on-year is Lewis, because he can pick himself up and literally change a lot about himself.
"It doesn't matter if it's training hard, or changing your diet, or changing how you're doing things psychologically. He would adapt every year and grow because he knew if you stayed the same, you wouldn't be successful the following year.
"He wasn't satisfied with being seven-time world champion. It's not a numbers game with him. He's personally pushing himself more and more to the limit, but that culture is the reason why Mercedes went on to win eight [titles]."
Even though Vowles and his colleagues very much enjoyed all the success they had in that period, he went on to describe the toll it took. He added: "It's really hard to win eight You have to keep everyone aligned with the direction of travel you're going in and it's tiring. You've got to maintain a level of operation that is just incredibly high.
"There can't be a second of let-down in order to keep going. It wears you down more and more. You don't reset, it just keeps wearing you down. You can't celebrate the now. You can't celebrate the success you have, because if you do, you fall behind.
"It's tremendously difficult but Lewis emanates that all the time. If you look at what he is, he's never the same individual when he comes back. I learned a tremendous amount from him, but all of them have something really special to them. That's why they're world champions."
